I'd suggest two things, one is to download some sounds of cars from the internet (or record them yourself), and play them over and over so she disassociate the sound of a car with the movement so won't be looking out for cars when she hears them.
This sounds like great advice! Honey likes to race cars along the fence line, she lines herself up when she hears them.
As for those comfort harnesses, we had one for Honey and she loathed it, she would run from it, so if you continue to have problems you might want to try another brand.
With Honey to keep her in the practice of minding us, I make her wait for me to enter or exit a door before she can follow, and once your pup has a good stay down, you can make her wait for your signal to exit...I've found those exercises and making Honey sit calmly before she gets food to really help with her paying attention to me when I give her commands, or need her attention.
It's something you can keep going back to as well, Honey's four and I still make her do it periodically.